Pneumatically driven small diameter piston structure

ABSTRACT

There is provided an improved structure for a small diameter piston used in a pneumatically driven free-piston vibrationinducing device. Such devices are subject to &#39;&#39;&#39;&#39;surging&#39;&#39;&#39;&#39; or variation in frequency. &#39;&#39;&#39;&#39;Surging&#39;&#39;&#39;&#39; is controlled or eliminated in such pistons by including means for restricting the piston surface area at minimum clearance from the cylinder wall, e.g. the provision of a peripheral groove or recess.

United States Patent [72] Inventor Warren C. Burgess, Jr. 760,088 5/1904 Tonjes 91/234 Avon Lake,0hlo 1,599,299 9/1926 Stoney..... 91/234 [21] Appl. No. 835,003 615,245 12/1898 Browne 91/50 [22] Filed June 20, 1969 1,535,659 4/1925 Fog 92/151 [45] Patented Aug. 24, I971 FOREIGN PATENTS [73] m 337,157 4/1959 Switzerland... 91/234 777 2 759,573 11/1933 France 91/234 7 v 1 1. 3 Primary Examiner-Paul E. Maslousky [54] PNEUMATICALLY DRIVEN SMALL DIAMETER Attorney-McNenny, F arrington, Pearne and Gordon PISTON STRUCTURE 5Cla1m ZDraw' Figs.

mg ABSTRACT: There is provided an improved structure for a [52] U.S.Cl 91/234 Small diameter piston used in a pneumatically driven free [5 I] CL 21/02 piston vibration-inducing device. Such devices are subject to [50] Field of Search 91/234 or variation in frequency. s is controlled or 56] References Cited eliminated in such pistons by including means for restricting the p1ston surface area at m1n1mum clearance from the UNITED STATES PATENTS cylinder wall, e.g. the provision of a peripheral groove or 542,498 7/1895 Rinsche 91/234 recess.

=---7 2e 3e so VIII Patented Aug. 24, 1971 3,601,009

INVENTUR WARREN C. BURGESS, JR

PNEUMATICALLY DRIVEN SMALL DIAMETER PISTON STRUCTURE BACKGROUND AND PRIOR ART It has been found that with pneumatically driven free-piston vibration-inducing devices a problem 'known as surging" occurs when the ratio of the circumference of the piston to the area at the end of the piston is 4 or greater. Surging is manifested by a change in frequency of oscillation occurring in the system without change of externally controllable parameters such as air pressure or the mass of the body being vibrated. Very often, such surging" is sufficient to throw a device being vibrated by such vibrationinducing devices out of synchronism with natural frequency. Why this change in frequency occurs is not well understood. It may be due to a change in the amount of water vapor in the gas being fed to the piston-cylinder assembly. Or, it may be due to the deposition of a film of oil in the clearance between the surface of the cylinder wall and the surface of the oscillating piston which film increases the drag on the piston. This problem is peculiar to small diameter pistons in which the ratio of the circumference to the cross-sectional area of the piston at full diameter is 4 or greater. The ratio of circumference to areafor a 1- inch diameter piston is 4. For a 0.75-inch diameter piston the ratio is 5.33. For a 0.50-inch diameter piston the ratio is 8. With smaller diameters, this ratio increases.

A particular problem with changes in frequency of vibration occasioned by surging" is that the system which is being driven, e.g. a vibratory parts-feeding bowl, acts as a multiplier or amplifier of the discontinuities introduced by changes in frequency. This is of particular importance in the handling of miniaturized parts. Devices capable of feeding miniaturized parts by vibratory means utilize small diameter free-piston vibration-inducing devices, for example those having a diameter less than 0.75 inch. In such cases, a surge may cause an amplitude variation in the device being driven by as much as 3 to times the tolerances acceptable by the tooling in the apparatus. This variation may cause a part to bypass tooling which might otherwise reject the part, thereby allowing a misoriented or otherwise rejectable part to enter the system. Such a part proceeding in the system frequently causes difficulty farther down the line. Such malfunctions must be minimized or eliminated insofar as possible.

Reference may be had to my prior U.S. Pat. No. 2,861,548 dated Nov. 25, 1958, for a disclosure of pneumatically driven free-piston vibration-inducing devices of the type in which surging can be controlled by the present invention. The disclosure of this patent is incorporated herein in its entirety by reference thereto. Reference may also be had to U.S. Pat. No. 3,023,738 dated Mar. 6, 1962, for teaching of the manner of using free-piston vibration-inducing devices in a particularly efficient manner. The disclosure of this patent is also incorporated herein in its entirety by reference thereto.

BRIEF STATEMENT OF THE INVENTION It has been found that surging in small pneumatically driven free pistons may be reduced or eliminated by restricting the piston surface area which is at minimum clearance from the cylinder wall. In the manufacture of pneumatically driven piston-cylinder structures, the clearances between the free pistons and the cylinder walls are those normally used for sleeve-type bearings, for example, from 0.001 to 0.003 inch per inch of diameter. With the small diameter pistons, i.e. those less than 1 inch in diameter, clearances are of the order of 0.002 inch to 0.0005 inch. It can be seen, therefore, that if water or oil enters the clearance between the cylinder ID. and the piston O.D. when clearances are of this magnitude, a viscous drag may occur, resulting in a change in frequency which will persist until the condition is relieved. In order to achieve peak performance, as diameter decreases, so also does the clearance. Thus the coefficient of drag due to viscous velocity profiles can be expected to increase much more easily with transient changes in vapor or fluid present in droplet form in the compressed actuating gas, for example compressed air.

By reducing the surface area of the piston at minimum clearance from the ID. of the cylinder, surging is controlled or eliminated.

BRIEF DESCRIPTION OF THE DRAWINGS In the annexed drawings:

FIG. 1 is a cross-sectional view of a free-piston vibration-inducing device of the type shown in the aforesaid U.S. Pat. No. 2,861,548 and having a piston therein modified in accordance with the present invention.

FIG. 2 is a side elevation of a typical piston modified in accordance with the present invention.

DETAILED DESCRIPTION OF DRAWINGS Referring now more particularly to FIG. 1, there'is here shown in cross section a typical free-piston vibration-inducing device adapted to be operated by a compressed gas, e.g. compressed air. This device consists of a casing or housing 10 having an inlet 12 for the compressed gas and exhaust ports 14 and 16. Inlet 12 is centrally located with respect to the free with internal ducts 26 and 28 extending in an axial direction and opening through the ends 30 and 32 of piston 24, respectively. Internal duct 26 communicates with radially extending duct 34 leading to the peripheral surface of the piston 24. In like manner radial duct 36 communicates between the peripheral surface of the piston 24 and internal duct 28. The

lengths of ducts 26 and 28 are greater than one-half the length of the piston whereby the radial ducts 34 and 36 are in staggered relation to each other. Radial ducts '34 and 36 are adapted for alternating gas-receiving communication with the circumferential groove 18 as the piston oscillates within the cylinder. The operation of the piston in the cylinder is fully described in the aforesaid U.S. Pat. No. 2,861 ,548.

Referring now more particularly to FIG. 2, there is shown in side elevation piston 24. Piston 24 is, geometrically speaking, configured generally as a right cylinder and is provided with end lands 38 and 40, and a centrally located intermediate land 42. End lands 38 and 40, and intermediate land 42, are at "full diameter, for example, in an illustrative embodiment, 0.4904 inch $00004 inch. Intermediate each of the end lands 38 and 40, and the central or intermediate land 42, respectively, there are provided recesses or grooves 44 and 46, respectively. The recesses or grooves 44 and 46 are ground to an CD. of 0.486 10.002 inch. In this example, therefore, it will be seen that the depth of the recess is of the order of from 0.0010 inch to 0.0034 inch. The ends of the piston 24 are conveniently provided with bosses 48 and 50 for best design and prevention of damage to the piston in event of impacting against the closure plugs 20 and 22, for example. The width of the end lands in I the illustrative embodiment is 0.05 inch. The width of the recesses 44 and 46 measured in an axial direction in the illustrative embodiment is 0.20 inch. The central or intermediate land 42 in the illustrative embodiment is 0.45 inch. The center line of each radial duct 34 and 36 is 0.609 inch from the more remote end, and the diameter of the radial ducts and the axial ducts is desirably 0.094 inch. The overall length of piston 24 in the illustrative embodiment is 1 1/32 inch.

The foregoing dimensions relate to a 0.5-inch piston which has been found substantially free of surging. When a 0.5-inch diameter piston is operated under the same conditions but having a right cylindrical structure such as shown in US. Pat. No. 2,861,548 with air containing moisture or oil, surging is experienced from time to time.

By the simple expedient of reducing the piston surface area which is at minimum clearance from the cylinder wall as by grinding recesses 44 and 46 adjacent each extremity of the piston, surging is minimized or eliminated as a problem.

Instead of a single recess such as shown in FIG. 2 adjacent each extremity, the reduction of the area at minimum clearance from the cylinder wall may be achieved by forming a plurality of recesses whereby the piston will contain a larger number of lands than shown in FIG. 2. Alternatively, helical grooves such as would be provided by cutting thread grooves in the surface would achieve the same result. Thus, any suitable means of reducing the surface area at minimum clearance from the internal surface of the cylinder in housing may be utilized to obviate the problem of surging in pneumatically driven free-piston vibration-inducing devices.

What is claimed is:

1. In a pneumatic free-piston vibration-inducing device including a cylinder closed at each end and a free generally right cylindrically shaped piston oscillatable therein wherein the ratio of the circumference to the cross-sectional area of the piston at full diameter is greater than 4, means in said piston for distributing gas under pressure to the ends of said piston from spaced intermediate peripheral points, gas inlet means coacting with said gas distributing means, and gas exhaust means extending through said cylinder, the improvement which comprises a piston including axially spaced recesses adjacent each end of said piston for restricting the piston surface area at minimum clearance from the cylinder wall, the depth of each recess being of the order of 0.0010 inch to 0.0034 inch.

2. In a pneumatic, free-piston vibration-inducing device including a cylinder closed at each end and a free generally cylindrically shaped piston oscillatable therein wherein the ratio of the circumference to the cross-sectional area of the portion of full diameter is greater than 4, means in said piston for distributing gas under pressure to the ends of said piston from spaced intermediate peripheral points, gas inlet means, and gas exhaust means, the improvement which comprises a piston characterized by a full diameter central portion, a full diameter land adjacent each end of the piston, and a portion intermediate each end land and said central portion having a diameter less than the full diameter of said piston by from 0.0020 inch to 0.0068 inch.

3. A pneumatic free-piston vibration-inducing device in accordance with claim 2 in which the intermediate portion is a circumferential groove.

4. A pneumatic free-piston vibration-inducing device in accordance with claim 3 in which the axial width of each end land is from one-half to one-tenth of the axial width of the circumferential groove.

5. A pneumatic free piston in accordance with claim 2 wherein the ratio of the circumference to the cross-sectional area of the piston at full diameter is greater than 8. 

1. In a pneumatic free-piston vibration-inducing device including a cylinder closed at each end and a free generally right cylindrically shaped piston oscillatable therein wherein the ratio of the circumference to the cross-sectional area of the piston at full diameter is greater than 4, means in said piston for distributing gas under pressure to the ends of said piston from spaced intermediate peripheral points, gas inlet means coacting with said gas distributing means, and gas exhaust means extending through said cylinder, the improvement which comprises a piston including axially spaced recesses adjacent each end of said piston for restricting the piston surface area at minimum clearance from the cylinder wall, the depth of each recess being of the order of 0.0010 inch to 0.0034 inch.
 2. In a pneumatic, free-piston vibration-inducing device including a cylinder closed at each end and a free generally cylindrically shaped piston oscillatable therein wherein the ratio of the circumference to the cross-sectional area of the portion of full diameter is greater than 4, means in said piston for distributing gas under pressure to the ends of said piston from spaced intermediate peripheral points, gas inlet means, and gas exhaust means, the improvement which comprises a piston characterized by a full diameter central portion, a full diameter land adjacent each end of the piston, and a portion intermediate each end land and said central portion having a diAmeter less than the full diameter of said piston by from 0.0020 inch to 0.0068 inch.
 3. A pneumatic free-piston vibration-inducing device in accordance with claim 2 in which the intermediate portion is a circumferential groove.
 4. A pneumatic free-piston vibration-inducing device in accordance with claim 3 in which the axial width of each end land is from one-half to one-tenth of the axial width of the circumferential groove.
 5. A pneumatic free piston in accordance with claim 2 wherein the ratio of the circumference to the cross-sectional area of the piston at full diameter is greater than
 8. 